DESCRIPTION: Mechanisms responsible for no susceptibility to infection with Schistosoma mansoni in Biomphalaria glabrata snails are not well understood. The most likely explanation for non-susceptibility is immunological resistance, possibly involving (1) recognizing the parasite as non-self and/or failing to recognize the parasite as self, (2) mounting a cytotoxic response against the sporocyst, as well as (3) withstanding or countering parasite immunosuppressive factors, which could interfere with (1) or (2). Both cellular and humoral mechanisms (which may act independently or in concert with hemocytes) have been implicated in sporocyst destruction. This resistance can be transferred to non-susceptible snails both adoptively, by transplants of the hematopoietic amebocyte-producing organ (APO) or passively, by injections of plasma. Although passive transfer was described over 16 years ago, it has remained uninvestigated, despite the potential for elucidating humoral mechanisms of schistosome resistance. Preliminary studies have confirmed the presence of resistance factor in plasma from additional types of resistant snails, and have provided new information on the duration of the effect, the fate of sporocysts in snails with passively transferred resistance, as well as the resistance factor molecular weight and source. This proposal seeks to further characterize passively transferred resistance to S. mansoni and to identify the relevant plasma resistance factor(s).